1. Field of the Invention
The present invention relates to a lithographic apparatus and a method to handle a closing element of the lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system, i.e. the bottom of the projection system, and the substrate. This enables more accurate projections and imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the immersion liquid may also be regarded as increasing the effective numerical aperture number NA of the system and also increasing the depth of focus. Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein. Thus, a lithographic apparatus may be provided with a fluid provider which is arranged to provide the immersion liquid, or to keep the liquid in its place. The liquid may be flowing to avoid local heating.
The substrate, or the substrate and the substrate table, may be immersed in a bath of immersion liquid. An example of such an arrangement is disclosed in U.S. Pat. No. 4,509,852 which hereby is incorporated by reference in its entirety. Alternatively, the immersion liquid may be provided by a liquid supply system only on a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system, the substrate generally having a larger surface area than the final element of the projection system. An example of such an arrangement is disclosed in International Patent Application No. 99/49,504 which hereby is incorporated by reference in its entirety. The liquid is supplied by at least one inlet on the substrate, preferably along a direction of movement of the substrate relative to the final element of the projection system, and the liquid is discharged by at least one outlet which may be connected to a low pressure source. Various orientations and numbers of inlets and outlets positioned near the periphery of the final element are possible. Further, a liquid supply system may be provided with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (the direction of the optical axis of the projection system). A seal is formed between the seal member and the surface of the substrate. Preferably the seal is a contactless seal such as a gas seal, which may further function as a gas bearing. An example of such an arrangement is disclosed in European Patent Application No. 03252955.4 which hereby is incorporated by reference in its entirety.
European Patent Application No. 03257072.3, which hereby is incorporated by reference in its entirety, discloses a twin or dual stage immersion lithography apparatus. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without the presence of an immersion liquid, and exposure is carried out with a stage at a second position, where an immersion liquid is present. Alternatively, the apparatus has only one stage.
A closing disk may have been provided to close an underside of the liquid supply system when e.g. the substrate is taken away, thereby preventing the immersion liquid to flow away. In a current implementation, the closing disc is held by a holder such as a pocket which may be located in an outside area of a top surface of the wafer stage or substrate table. When illumination of a substrate has taken place, the substrate table may be moved such that the liquid supply system faces the closing disc, thus in other words is positioned over the closing disc by moving the substrate table or stage with respect to the liquid supply system. The closing disc may now be taken out of the holder by the liquid supply system, the closing disc thereby preventing a flowing away of the immersion liquid held by the liquid supply system, as an underside of the liquid supply system is closed thereby. The substrate table may now be taken away from the liquid supply system, to e.g. change the substrate, etc. The closing disc may be held by the liquid supply system by means of under pressure such as by means of vacuum suction. When a following substrate it to be illuminated by the lithographic apparatus, the closing disc may be guided back to the holder, e.g. by moving the substrate table/substrate stage and the liquid supply system with respect to each other such that the closing disc is brought back to the holder again. An issue which comes forward now is that in current implementations, a size of the holder is only marginally larger than a size of the closing disc to keep a gap between the closing disc and the surface of the substrate table/stage to a minimum. This gap is to be kept to a minimum as thereby a leakage of the immersion liquid between the surface of the substrate table/stage and the closing disc may be prevented to a large extent when the substrate table/stage is moved with respect to the liquid supply system such that the position of the liquid supply system changes from facing the substrate to facing the closing disc, whereby the gap between the substrate table and the closing disc is to be passed. Due to this small cap, an accurate positioning of the closing disc is required when the closing disc is positioned back into the holder again. According to the state of the art, this is performed by slowly bringing the closing disc (held by the liquid supply system) and the holder which forms part of the substrate table/stage together until a mechanical contact is established between the closing disc and the holder. Additionally, a position of the closing disc is according to the state of the art measured periodically, e.g. a measurement being performed after a predetermined number of substrates have been illuminated by the lithographic apparatus, an example of such number being 100 illuminations. Such a position measurement is in the state of the art performed by an optical measurement, in particular in that a marker has been located on the closing disc, an optical measurement being performed by the projection system to detect a position of the marker. In a current implementation, the marker comprises a transmission image sensor (TIS) marker, the measurement system comprising a transmission image sensor measurement system. By the measurement, a deviation of an expected position of the closing disc may be corrected. The optical measurement requires a relatively long measurement time, and therefore performing the measurement more frequently would further reduce a speed of the lithographic apparatus.
One of the requirements for future generations of lithographic apparatuses is to increase a number of substrates of wafers that the lithographic apparatus is able to process in a certain time span, in other words to increase a speed or yield of the lithographic apparatus. Efforts are spent to increase a scanning of the substrate, improve accuracy to prevent incorrect illuminations of the substrate, etc. From this view point, the current bringing back of the closing disc to the holder at a low speed may be desirable.